Interpretive Summary: Methyl bromide has been used for decades as a soil fumigant for the control of nematodes, weeds and fungi and has recently come under scrutiny as a chemical which depletes stratospheric ozone. As a result of the Clean Air Act, methyl bromide is schedule for phase-out by the year 2001. Restricting the use of methyl bromide will cause substantial adverse economic impacts on the agricultural community. Therefore, a research project was initiated to determine how much methyl bromide enters the atmosphere after application. To accomplish this, a field experiment was conducted to measure the methyl bromide movement from the soil into the atmosphere after being applied at a shallow depth under a 1-mil polyethylene tarp. A significant difference between this experiment and previous experiments measuring methyl bromide movement is the inclusion of a mass balance. Without this vital information, it becomes impossible to determine the accuracy of the measured values.

Technical Abstract:
An experiment to investigate the environmental fate and transport of methyl bromide in agricultural field is described. The methyl bromide volatilization rate was determined as a function of time for conditions where methyl bromide was applied at a rate of 843 kg in a 3.5-ha (i.e., 240 kg/ha) field covered with plastic at a depth of 25 cm. Three methods were used to estimate the methyl bromide volatilization rate, including: the aerodynamic, theoretical profile shape and integrated horizontal flux methods. The highest methyl bromide volatilization rates were at the beginning of the experiment. Within the first 24 hours approximately 36% of the applied methyl bromide mass was lost. Diurnally, the largest volatilization rates occurred during the day when temperatures were high and the atmosphere was unstable. Cooler temperatures, light winds and neutral to stable atmospheric conditions were present at night; reducing the flux. The total emission calculated using these methods was found to be approximately 64% (+/- 10%) of the applied mass. A mass balance was calculated using each flux estimation technique and several methods for analyzing data. The average mass recovery using all the flux methods was 867 kg (+/- 83 kg) which was 102.8% (+/- 9.8%) of the applied mass (i.e., 843 kg). The range in the mass balance percent (i.e., percent of the applied mass that is measured) is from 88% to 112%. The averaged mass balance percent for the aerodyamic method, which involved using the measured data directly, was approximately 100.8%. The total emission calculated using the aerodynamic method was found to be approximately 62% (+/- 11%) of the applied mass.